This invention relates to a method for measuring ferromagnetic objects embedded in nonmagnetic materials. More specifically the invention relates to a method for measuring the diameter of reinforcement steel embedded in concrete and the dimension of the concrete overlayer by measuring and evaluating the effect of the ferromagnetic objects on a magnetic field. The invention further relates to an apparatus for practicing the method.
European Patent No. EP-A1-0 080 676 describes a test instrument for determining the dimension of the concrete overlayer which makes it possible, in the inspection of steel-reinforced concrete structures or concrete components, to determine that the specifications for the concrete overlayer have been met. This prior art test instrument for measuring the concrete overlayer utilizes the attraction force of a permanent magnet or a permanent magnet system, which force varies as a function of the spacing from the ferromagnetic parts. However, this method requires initial calibration of the test instrument on a specified concrete overlayer. Therefore, the use of such prior art test instruments is inexact and results in significant measuring errors.
Prior art instruments based on the principle of attentuating a parallel oscillatory circuit have also been provided for measuring the concrete overlayer and for estimating the bar diameter. These instruments pass an alternating current of a specific frequency through the coil of a probe which produces a magnetic alternating field. Metallic objects located within the effective range of the alternating field cause a change of the coil voltage as a function of the dimension of the overlayer and bar diameters. Such a reinforcement instrument, described for instance in a brochure by Proceq S.A. of Zurich, Switzerland, results in significant measuring errors in the case of unknown bar diameters. It is also possible to measure the overlayer with the aid of magnetic resistance measurement, if the diameter of the steel reinforcement is known. An eddy current impedance measurement employing empiric calibration involves problems due to the large fluctuations of the permability of steel.
In view of this state of the prior art, it is an object of the present invention to provide a method and an apparatus to determine, in addition to the dimension of the overlayer, the diameter of the steel reinforcement without the need for a sample for calibration or comparison purposes.